Atomic Structure and Radioactivity

Structure of the Atom

 Dalton states that every substance is composed of indivisible particles called atoms.

 Atoms of one element are all alike and have the same atoms.

 Atoms are particles of elements or substance. These cannot be broken further without changing the chemical nature of the substance.

Rutherford’s and Bohr’s postulations

 Atomic structure of an atom mainly consists of three particles such as electrons, protons and neutrons.

 These sub-atomic particles have different properties. The centre of the atom has nucleus which consists of proton and neutrons.

 Protons have positive electric charge and it is much larger and heavier than electrons.

 Neutrons have no electrical charges and it is much heavier and larger like protons.

 Electrons are very small, tiny, and lighter particles and it has negative electric charge.

 Electrons are present at considerable distance from the nucleus in a series of levels called energy levels.

 Each energy level can hold certain number of electrons only. The first energy level which is very

close to the nucleus can hold only 2 electrons, the second and third level can hold only 8 and 18 electrons respectively.

 The atom can either gain or lose electrons to become negatively or positively charged.

 The atoms which have 1, 2, or 3 electrons present in outermost orbital can lose the electrons while interaction with atoms which have 5, 6, or 7 electrons present in the outermost orbital.

 The atoms which have 5, 6, or 7 electrons in the outermost orbital can gain the electrons while

interaction with atoms which have 1, 2, or 3 electrons in the outermost orbital.

 The atoms which have 4 electrons in the outermost orbital will tend neither to totally lose nor totally gain electrons during interactions.

Cathode Rays

 J.J. Thomson is created with discovery of electrons.

 Cathode rays consist of negatively charged particles called electrons.

 They originate from cathode and travel through straight line.

 These rays deflect towards POSITIVE

 The nature of the cathode rays does not depend on the nature of the gas inside the tube or the cathode used.

 The ratio of the charge to mass (e/m ratio) of cathode ray particles obtained from different gases was found to be exactly the same.

Anode Rays

 Goldstein experimentally proved the existence of positive charge in matter.

 These rays consist of positively charged particles to balance the negatively charged electrons.

 They do not originate from anode.

 These rays deflect towards the negative plate of an electric field.

 The charge to mass (e/m) ratio of the particles in anode rays depends upon the nature of the gas taken in the discharge tube.

 In the case of hydrogen, the e/m ratio is the highest as the positive particles obtained from hydrogen are the lightest.

 A hydrogen-1 atom has only one Proton, one Electron, and no neutrons.

 Isotopes of hydrogen:

 Protium-1 proton, 0 neutrons

 Tritium-1 proton, 2 neutrons

 An atom that has neither a positive nor a negative charge is neutral. That atom is said to have a neutral charge.

 An atom becomes negatively charged by addition of an extra electron orbiting the nucleus.

 An atom becomes positively charged if an extra proton joins the nucleus.

Atomic Number

 The number of protons present in the atom determines the atomic number of atom.

Atomic number = Number of protons present in nucleus = Number of electrons

Atomic mass = Number of protons + Number of neutrons

 The elements which are having same number of atomic number and varying mass number are called Isotopes.

 The elements which are having same mass number and varying atomic number are called isobars.

Mass Number (A)

 The mass number A of an atom is the total number of its nuclear constituents, or nucleons as the proton and neutrons are collectively called.

Isotopes

 Isotopes are atoms of the same element that have different mass due to differences in the number of neutrons they contain.

 Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number, thus: Ag. 

They may also be designated with a subscript notation indicating the number of neutrons.

 Many isotopes are stable, meaning that they are not subject to radioactive decay, but many more are radioactive.

 The latter, also known as radioisotopes, play a significant role in modern life.

 Carbon-14, for instance, is used for estimating the age of objects within a relatively recent span

of time-up to about 5,000 years-whereas geologist and other scientists use uranium-238 to date

minerals of an age on a scale with the most stable isotopes is easy to remember because its name is almost the same as its number of stable isotopes: tin, with 10.

Radioactivity

 Natural radioactivity was first observed in 1896 by A.H. Becquerel but the term radioactivity was coined by Madam Curie.

 Radioactive decay is the spontaneous radioactive disintegration of an atomic nucleus resulting in the release of energy in the form of particles.

 Radioactive decay is one process through which unstable atoms can become more stable.

 The emissions from an unstable atom’s nucleus, as it decays, can be in the form of alpha, beta or gamma radiation.

Alpha Particles

 In alpha decay, a positively-charged particle is emitted from the nucleus of an atom.

 Alpha particles are made of 2 protons and 2 neutrons. This means that they have a charge of +2, and a mass of 4.

 Alpha particles are relatively slow and heavy.

 They have a low penetrating power you can stop them with just a sheet of paper.

 Because they have a large charge, alpha particles ionize other atoms strongly.

Beta Particles

 In beta decay, a particle is emitted from the nucleus of an atom.

 Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton.

 This means that beta particles are the same as an electron.

 They are fast, and light.

 Beta particles have a medium penetrating power-they are stopped by a sheet of aluminium or plastics such as Perspex.

 Beta particles ionize atoms that they pass, but not as strongly as alpha particles do.

Gamma Rays

 Gamma decay occurs because the nucleus of an atom is at too high as energy state.

 Gamma rays are waves, not particles. This means that they have no mass and no charge. So we sometimes write.

 Gamma rays have a high penetrating power-it takes a thick sheet of metal such as lead, or concrete to stop them significantly.

 Gamma rays do not directly ionize other atoms, although they may cause atoms to emit other particles which will then cause ionization.

 We don’t find pure gamma sources – gamma rays are emitted alongside alpha or beta particles.

Nuclear Fission

 An atom’s nucleus can be split apart. When this is done, a tremendous amount of energy is released in the form of both heat and light.

 Inside a nuclear reactor, uranium atoms are split apart in a controlled chain reaction. 

 In a chain reaction, neutrons released by splitting of the atom off and strike other uranium atoms, splitting those and releasing another lot of neutrons to continue striking more uranium atoms. In nuclear reactors, design features and control roads are used to regulate the splitting so it does not go too fast.

 If the reaction is not controlled and the fuel is almost pure uranium-235 or plutonium, you could have an atomic bomb. These conditions are not present in a nuclear reactor.

Nuclear Reactor

 It is a device that can initiate and control a self-sustaining series of nuclear-fission reactions.

 Nuclear reactor is a complex device in which fissionable elements such as uranium, thorium, or plutonium are made to undergo a sustainable nuclear chain reaction.

 This chain reaction release energy in the form of radiation that (a) sustains the chain reaction;

(b) transmutes (i.e., alters the nuclear characteristics of nearby atoms, including the nuclear fuel itself) and (c) may be harvested as heat.

 The control rods such as cadmium or boron are lowered into nuclear material to absorb the neutrons thus slowing the reaction or raised to allow more reaction.

 Materials used as moderators include heavy water, graphite, beryllium and certain organic compounds.

 A thermal nuclear reactor is the one where the majority of fissions are caused by slow neutrons.

 A fast nuclear reactor is the one where the majority of fissions are caused by fast neutrons.

 The breeder reactor is a special type used to produce more fissionable atoms than it consumes.

Nuclear Fusion

 Fusion: it involves joining of smaller nuclei by nuclear fusion of hydrogen atoms into helium atoms. This gives off heat, light and other radiations.

 Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity.

Radioisotopes

 Radioisotopes are radioactive isotopes of an element.

 Different isotopes of the same element have the same number of protons in their atomic nuclei but different numbers of neutrons.

 They can also be defined as atoms that contain an unstable combination of neutrons and protons. (nucleus)